DAKP has non-standard "source" format

[DnlRKorn]

I noticed this while doing ingest summarization. In the DAKP edges, the information on knowledge_source are stored under the "sources" key are stored as a list of specialized objects; while in most of the other KGX edge files, it is stored as a string under "primary_knowledge_source", "secondary_knowledge_source" and "aggregator_knowledge_source".

This may be intentional to capture the fact that DAKP has multiple secondary knowledge sources, but I wanted to confirm.